It's mainly the small combs and wattles, and lack of any redness in those areas. At 8 to 9 weeks, a cockerel would have more comb and wattle development.
One of the things to check is the skin at the bottom of their feet. If this is white or pinkish, then perhaps they are Marans. If the skin in that area is yellowish, then maybe two production blues and two black sex links.
Feathered legs are another thing to look at-- Marans often have them...